1. Field of the Invention
This invention relates to a specific thermoplastic polymer composition which comprises an ethylene-propylene rubber, an ethylene copolymer, a propylene polymer (including a propylene-ethylene block copolymer) and talc, shows good processability on injection molding, can give a molded product having a good appearance, is excellent in the surface hardness, impact resistance and adhesion of coating, and thus is suited for the production of injection-molded products such as automobile parts.
2. Background Art
Heretofore, many attempts have been made to enhance the value of molded products of various rubbers such as an ethylene-propylene copolymer rubber by improving their fluidity and imparting rigidity to them. For instance, compositions prepared by incorporating polypropylene to rubber have been known as disclosed in Japanese Patent Publications Nos. 57-57049, 62-5460 and 62-5461. However, the proportion of polypropylene in these compositions is generally small, and polypropylene having high fluidity and high crystallinity is not particularly employed, so that the compositions have low crystallization rates. A long cooling time is therefore required when large-sized molded products of the compositions are produced by means of injection molding. The productivity is thus extremely low. In addition, since the compositions contain neither ethylene copolymers nor talc, they can give only such molded products that are poor in the surface smoothness and surface hardness.
The composition disclosed in Japanese Patent Publication No. 61-19651 comprises a relatively large amount of polypropylene. However, this composition also contains neither ethylene copolymers nor talc. Moreover, a partially crosslinked rubber is used as a rubber component. For these reasons, the composition is also confronted with the same problems as in the above compositions in the productivity upon producing molded products, and in the surface smoothness and surface hardness of molded products.
On the other hand, Japanese Patent Publication No. 60-3420 discloses a composition prepared by incorporating an ethylene-propylene rubber and talc into a propylene-ethylene block copolymer. This composition is excellent in adhesion of coating and low-temperature impact resistance. Further, Japanese Patent Laid-Open Publication No. 1-204946 discloses a composition comprising an ethylene-propylene rubber, a propylene-ethylene block copolymer, an ethylene copolymer and talc. This composition can give molded products having an improved dimensional stability. These compositions are, however, still insufficient in the surface hardness and smoothness of molded products. The molded products will be easily flawed due to their low surface hardness and have a poor appearance due to their poor surface smoothness.
It is therefore an object of the present invention to solve the above problems in the prior art and provide a thermoplastic polymer composition which shows good processability on injection molding, can give an injection-molded product having a good appearance and a low density, and is excellent in the surface hardness, impact resistance and adhesion of coating.